The move was confirmed by the Saints on Twitter, where they announced the four-year deal:
Hull boss Steve Bruce earlier explained to the Hull Daily Mail that Southampton's £12.5 million offer was too good to turn down:
We'd not been looking to sell Shane but it's a fantastic offer and we've decided to accept it.
The challenge is always to become bigger and better and this money will be put towards strengthening again.
Whether you are Cristiano Ronaldo or Lionel Messi, everyone in football has got their price.
Having lost the likes of Rickie Lambert, Adam Lallana and Dejan Lovren to Liverpool, and Luke Shaw to Manchester United, the Saints are in need of a major rebuild if they wish to repeat last season's impressive eighth-place Premier League finish.
Long joins Jay Rodriguez and new signing Graziano Pelle as part of Southampton's strike force. New boss Ronald Koeman likely sees him as an effective replacement for Lambert.
However, the 27-year-old Long will need to improve on his return from last season if he is to truly make an impact at St. Mary's. He scored a combined seven league goals in 30 appearances for West Brom and Hull.
Indeed, there are some who feel that Long's talents do not necessarily demand such a high transfer fee, including football presenter Jacqui Oatley and The Guardian's Ed Aarons:
However, Southampton are somewhat victims of their own success. They have very publicly come into some serious money over the transfer window with the numerous departures, making it difficult for them to drive a hard bargain.
Long only moved to Hull in January from West Brom, but the Tigers were clearly eager to cash in on the chance to make a profit, having bought him for an estimated £7 million.
While his goal return is not the best, he does have Premier League experience, which will be key for a Saints side likely to be full of new players in the coming season.
He also will be playing in a side where he gets better service than his previous clubs, given the presence of the likes of Rodriguez, James Ward-Prowse and Morgan Schneiderlin in Koeman's squad.
The move is a great opportunity for Long to play a part in an exciting side looking to prove many doubters wrong, but Southampton may live to regret the move if he cannot find the scoresheet more often.